Japanese Patent No. HEI 4-24937 describes a charging method in which an apparatus resumes charging in the event the terminal voltage of a lead acid secondary battery drops from a full charge level to a lower level as a result of self-discharge. In order to maintain the lead acid secondary battery at a full charge level, an apparatus in accordance with the method of this reference initially charges the secondary battery with a constant current, while monitoring the terminal voltage of the secondary battery. When the terminal voltage reaches a first predetermined voltage value corresponding to the full charge level charging is stopped. If the terminal voltage subsequently falls below a second predetermined voltage value, charging is resumed.
This method can maintain the secondary battery at or near the full charge level. However, when the apparatus charges a battery assembly which includes a plurality of secondary batteries and should one of the secondary batteries malfunction as a result of an inner short, for example, the remaining secondary batteries may become overcharged.
For example, a malfunctioning secondary battery can cause the terminal voltage of a battery assembly to drop immediately after being charged to a full charge level. Therefore, although the charge level of the other secondary batteries remain at or near a full charge level, the apparatus resumes charging, because the terminal voltage of the assembly falls below the second predetermined voltage value as a result of the malfunctioning secondary battery. By resuming charging, the other properly operating secondary batteries can become overcharged. Excessive overcharging can increase the internal gas pressure of the overcharged secondary batteries, causing a safety vent to open and thereby permit electrolyte inside the secondary batteries to leak through the open safety vent.